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Characteristics of special circulations.

Abstract

Blood flow through a vascular bed is usually determined by the pressure gradient across it and the diameter of the precapillary resistance vessels. Special circulations have additional specific features of blood flow control. Several organs control their blood supply by autoregulation. Coronary blood flow is linked to myocardial oxygen consumption, primarily by a metabolic mechanism. Increases in demand or decreases in supply of oxygen cause the release of vasodilator metabolites, which act on vascular smooth muscle to cause vessel relaxation and hence increase blood flow. Cerebral blood flow is primarily regulated by a myogenic mechanism whereby increases in transmural pressure stretch the smooth muscle, which responds by contracting. Renal blood flow is regulated by both extrinsic and intrinsic mechanisms; sympathetic vasoconstriction of the afferent arterioles reduces renal blood flow in response to a decrease in effective circulating volume, myogenic mechanisms and tubuloglomerular feedback, as well as the release of vasoactive metabolites from the vascular endothelium regulate renal blood flow intrinsically. Hepatic blood flow is delivered via the portal vein and hepatic artery, and the amount of flow varies in these vessels reciprocally to maintain constant total blood flow. The pulmonary circulation receives the entire cardiac output, and blood flow is regulated both passively and actively. Pulmonary vessels are highly distensible and can accommodate increases in blood flow without significant increases in pressure.